1. Field of the Invention
This invention relates to a coal combustion apparatus provided with a denitration means. More particularly it relates to a coal combustion apparatus devised so that a denitration catalyst can be hardly poisoned by volatile metal compounds contained in exhaust gases in a denitration means for catalytic reduction with ammonia, and a method for eliminating said volatile metal compounds from said exhaust gases.
2. Description of the Related Art
Heretofore, for removing nitrogen oxides contained in exhaust gases from a coal combustion apparatus such as boiler facilities, etc., a catalytic reduction process using ammonia as a reducing agent has been mainly employed. A model of boiler facilities provided with a denitration apparatus is shown in FIG. 2 of the accompanying drawings. The boiler facilities comprise a boiler furnace 2 provided with a coal-feeding flow path 1 and a slag-discharging flow path 9, and a denitration apparatus 3, an air preheater 5, an electrostatic precipitator 6 and a chimney 7, each successively provided in the exhaust gas flow path of the boiler furnace 2 and further a line 8 for recycling ashes collected in the electrostatic precipitator.
In the boiler facilities shown in FIG. 2, simple substances or oxides of highly volatile elements such as arsenic, selenium, lead, zinc, etc. volatize inside the boiler furnace 2, and are mostly adsorbed to fly ashes (powder of coal combustion ashes), before they are collected by the electrostatic precipitator 6.
The fly ashes having adsorbed the compounds of the volatile elements are blown into the boiler furnace via a fly ash-recycling path 8 and recovered through a slag-discharging path 9 to the outside, but the compounds of the volatile elements are left behind inside the boiler system in the form of vapor and present in the form of a highly concentrated vapor in the exhaust gas. When the denitration apparatus 3 is provided inside the flow path of the exhaust gas containing such a vapor of the volatile metal compounds, a denitration catalyst 4 in the denitration apparatus 3 adsorbs the highly concentrated volatile metal compounds to notably reduce its activity.
The present inventors have previously invented a denitration catalyst having small deterioration enough to be usable even in such boiler facilities and have applied for patent (Japanese patent application No. Sho 62-141176/1988), but nevertheless it is necessary to take some countermeasure for preventing the deterioration, in addition to the catalyst improvement.